This invention relates to the control of a bus line to which a plurality of integrated circuits are coupled, and more particularly to a means to control the logic level of the bus line when all integrated circuits outputs coupled to the bus line are in a default no op state.
Heretofore, communication by multiple integrated circuits along a common bus line have utilized open collector or tri-level (logic 0, logic 1, and high impedance) logic devices to couple to the bus line. To prevent spurious communications, some means of controlling the default state of the bus line must be provided so that when all devices are in a default no op state, the bus line will be at a known logic level. Heretofore, this problem has been solved by the use of pull up or pull down resistors, one end of the resistor coupled to the bus line and the other end of the resistor coupled to a power supply bus. This has the undesirable side effect of dissipating a large amount of power due to the resistive loss. Another solution has been to provide additional control lines to create a master/slave protocol. Thus, a protocol is established between devices where at least one device has control of the bus at all times, and where that one device relinquishes the bus line and another device assumes control of that bus line. This approach has the disadvantage of requiring additional control lines and additional control logic, thereby increasing the size of each integrated circuit in the system and increasing the complexity and size of the total system.